Journal of Geophysical Research: Planets最新文献

The polar vortices of Mars are characterized by strong zonal winds that isolate cold air above the pole, allowing CO₂ to condense out of the atmosphere through snowfall and direct deposition. Due to their key role in seasonal variability of the atmosphere, it is important to understand the different factors that affect the strength, shape, and stability of the polar vortices and processes such as snowfall that occur within. We used atmospheric retrievals of temperature and CO₂ ice cloud opacity from the Mars Climate Sounder (MCS) on board NASA's Mars Reconnaissance Orbiter (MRO) to characterize and analyze patterns in the polar vortices and CO₂ ice clouds for Mars years (MY) 29–36. We couple the MCS data with a one-dimensional snowfall model to determine CO₂ snow precipitation rates and analyze patterns in the amounts and distribution of snowfall. We characterize the elliptical nature of both vortices and find that there is significant shrinking and warming of the polar vortex during regional dust storms in the summer hemisphere, which occur more frequently during northern winter. We also find that snowfall in the north pole exceeds that in the south and accounts for ∼1% of surface CO₂ deposition, with a notable pause in snowfall during the solstice. We also find measurable variability in snowfall driven by both regional and global dust storms and persistent yearly patterns in the spatial distribution of snow clouds.

Interactions between magma oceans and overlying atmospheres on young rocky planets leads to an evolving feedback of outgassing, greenhouse forcing, and mantle melt fraction. Previous studies have predominantly focused on the solidification of oxidized Earth-similar planets, but the diversity in mean density and irradiation observed in the low-mass exoplanet census motivate exploration of strongly varying geochemical scenarios. We aim to explore how variable redox properties alter the duration of magma ocean solidification, the equilibrium thermodynamic state, melt fraction of the mantle, and atmospheric composition. We develop a 1D coupled interior-atmosphere model that can simulate the time-evolution of lava planets. This is applied across a grid of fixed redox states, orbital separations, hydrogen endowments, and C/H ratios around a Sun-like star. The composition of these atmospheres is highly variable before and during solidification. The evolutionary path of an Earth-like planet at 1 AU ranges between permanent magma ocean states and solidification within 1 Myr. Recently solidified planets typically host $H_{2}O$ - or $H_2$ -dominated atmospheres in the absence of escape. Orbital separation is the primary factor determining magma ocean evolution, followed by the total hydrogen endowment, mantle oxygen fugacity, and finally the planet's C/H ratio. Collisional absorption by $H_2$ induces a greenhouse effect which can prevent or stall magma ocean solidification. Through this effect, as well as the outgassing of other volatiles, geochemical properties exert significant control over the fate of magma oceans on rocky planets.

Valley networks (VNs) on Mars are crucial for understanding the Martian hydrologic and climatic history. However, the limited resolution of remote sensing data hindered the complete identification of Martian VNs, affecting our understanding of the formation and duration of VNs as well as their climatic significance. In this study, we utilized high-resolution imaging and topographic data to conduct detailed mapping and investigations of the VNs around the northwestern margin of the Hellas basin, the largest impact basin and major sedimentary sink in the Martian southern highlands. We identified a total of 911 VNs with a cumulative length of 32,086.3 km, more than twice that of previous mapping results. Additionally, we analyzed the morphological parameters of VNs, including stream order, sinuosity, junction angle, stream slope, etc., investigated their geomorphologic characteristics, and determined their formation ages. We propose that occasional precipitation and regional groundwater fostered the formation of well-developed VNs and a “Hellas Ocean” in the Noachian Period. The main fluvial activity occurred during ∼3.9–3.2 Ga. Subsequently, the climate transitioned from warm and semiarid to cold and arid during the Noachian-Hesperian transition, leading to the evaporation of the “Hellas Ocean.” In the Amazonian, some small simple valleys formed during ∼2.1–1.0 Ga with the supply of meltwater associated with obliquity-controlled glacial processes. These results reveal prolonged fluvial activity in the northwestern Hellas region with diverse water sources under changing climatic conditions, which make the region a very promising candidate for future in situ exploration missions.

The Apollo 17 73001/73002 double drive tube, collected at the base of the South Massif in the Taurus-Littrow Valley, was opened in 2019 as part of the Apollo Next Generation Sample Analysis program (ANGSA). A series of continuous thin sections were prepared capturing the full length of the upper portion of the double drive tube (73002). The aim of this study was to use Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN), to search for clasts of non-lunar meteoritic origin and to analyze the mineralogy and textures within the core. By highlighting mineral groups associated with meteoritic origins, we identified 232 clasts of interest. The elemental composition of 33 clasts was analyzed using electron microprobe analysis that revealed that all clasts were of lunar origin, suggesting that any meteoritic component in the regolith material we studied is not present in the form of lithic clasts. In the process of searching for meteorite fragments, we also identified a number of clast types including a group with highly magnesian olivine compositions (Fo_92.2-96.5). We extracted raw pixel data to investigate changes in mineralogy with depth, used QEMSCAN processors to separate and group individual clasts based on mineralogy, and determined variations in particle size with depth. Our results show a decreasing abundance of glass and agglutinate clasts with depth, associated with a higher soil maturity in the upper portion of the core. The lack of stratigraphy and dominance of non-mare clasts is consistent with the landslide origin of the material from the South Massif.

Detailed soil characterization at Gale crater based on in situ observations has revealed compositional trends within soils, while the physical and chemical processes underlying the compositional trends remain to be evaluated. Here we use the grain-morphometrical and geochemical trends across the Wentworth-classes of 48 in situ soil targets at Gale crater to evaluate underlying pedological processes and potential chemical weathering signatures. The concentration of olivine minerals within the ∼250 to ∼500 μm size range indicates the prevalence of heavy mineral sorting in a granulometric sense in Gale soils that surpasses the possible effect of the cratering-induced mixing processes. The extent of olivine sorting in soils varies spatially and is influenced by the targets' aeolian setting. The finest portion of Gale soils (<125 μm) is likely a mixture of allochthonous sediment, globally sourced from atmospheric suspension, and autochthonous counterparts from the weathering of local rocks, while the coarser soils (>125 μm) are mostly sourced from local rocks, with possible inputs from both the unaltered parent rock of the Stimson formation and the bedrocks that have undergone diagenetic alteration. If applicable globally, this would reinforce prior inferences that even dust-mantled regions are geochemically heterogeneous owing to a substantial fraction of soils derived from underlying lithology. The low chemical weathering intensity and coupling of mobile elements in soils suggest localized, low pH and low water-to-rock ratio aqueous weathering conditions under predominantly cold and arid climates, which may occur either during post-pedogenetic alteration in soils or during the acidic alteration of source rocks.

The motion of liquid iron (Fe) alloy materials in the outer core drives the dynamo, which generates Mercury's magnetic field. The assessment of core models requires laboratory measurements of the melting temperature of Fe alloys at high pressure. Here, we experimentally determined the melting curve of Fe9wt%Si and Fe17wt%Si up to 17 GPa using in situ and ex situ measurements of intermetallic fast diffusion that serves as the melting criterion in a large-volume press. Our determined melting slopes are comparable with previous studies up to about 17 GPa. However, when extrapolated, our melting slopes significantly deviate from previous studies at higher pressures. For Mercury's core with a model composition of Fe9wt%Si, the melting temperature-depth profile determined in our study is lower by ∼150–250 K when compared with theoretical calculations. Using the new melting curve of Fe9wt%Si and the electrical resistivity values from a previous study of Fe8.5wt%Si, we estimate that the electronic thermal conductivity of liquid Fe9wt%Si is 30 Wm⁻¹K⁻¹ at the Mercury's CMB pressure of 5 GPa and 37 Wm⁻¹K⁻¹ at an assumed ICB of 21 GPa, corresponding to heat flux values of 23 mWm⁻² and 32 mWm⁻², respectively. These values provide new constraints on the core models.

The Curiosity rover explored the region between the orbitally defined phyllosilicate-bearing Glen Torridon trough and the overlying layered sulfate-bearing unit, called the “clay-sulfate transition region.” Samples were drilled from the top of the fluviolacustrine Glasgow member of the Carolyn Shoemaker formation (CSf) to the eolian Contigo member of the Mirador formation (MIf) to assess in situ mineralogical changes with stratigraphic position. The Sample Analysis at Mars-Evolved Gas Analysis (SAM-EGA) instrument analyzed drilled samples within this region to constrain their volatile chemistry and mineralogy. Evolved H₂O consistent with nontronite was present in samples drilled in the Glasgow and Mercou members of the CSf but was generally absent in stratigraphically higher samples. SO₂ peaks consistent with Fe sulfate were detected in all samples, and SO₂ evolutions consistent with Mg sulfate were observed in most samples. CO₂ and CO evolutions were variable between samples and suggest contributions from adsorbed CO₂, carbonates, simple organic salts, and instrument background. The lack of NO and O₂ in the data suggest that oxychlorines and nitrates were absent or sparse, and evolved HCl was consistent with the presence of chlorides in all samples. The combined rover data sets suggest that sediments in the upper CSf and MIf may represent similar source material and were deposited in lacustrine and eolian environments, respectively. Rocks were subsequently altered in briny solutions with variable chemical compositions that resulted in the precipitation of sulfates, carbonates, and chlorides. The results suggest that the clay-sulfate transition records progressively drier surface depositional environments and saline diagenetic fluid, potentially impacting habitability.

Utilizing Mars Atmosphere and Volatile EvolutioN (MAVEN) observations from October 2014 to May 2023, we perform a detailed survey of magnetosonic waves generated in the solar wind (refer to as upstream MS waves), with frequencies near the proton gyrofrequency in the solar wind environment. The distribution of the solar wind-generated MS waves has been carefully investigated, including in the solar wind and in the Martian magnetosphere by propagation. The results show that these MS waves are widely distributed below the Martian bow shock but are more concentrated below the magnetic pileup boundary, particularly in the subsolar region. The waves possess higher occurrence rates on the dayside with larger amplitudes; the occurrence rates also show dusk-side-preferred asymmetry. The Martian crustal magnetic field can prevent MS waves from penetrating into lower altitudes, while higher solar dynamic pressure benefits their penetration. The wave amplitudes exhibit a weak positive correlation with the solar wind dynamic pressure. These obtained global distribution features of Martian upstream MS waves observed by MAVEN are valuable to improve current understanding of the dynamic variations of Martian charged particles and the underlying contribution of wave-particle interactions driven by MS waves.

Oxia Planum, Mars, is the future landing site of the ExoMars Rosalind Franklin rover mission, which will search for preserved biosignatures in a phyllosilicate-bearing unit. Overlying the mission-important phyllosilicate-bearing rocks is a dark, capping unit—known here as the Low albedo, Thin, Resistant (LTR) unit—which may have protected the phyllosilicate-bearing unit over geologic time from solar insolation and radiation. However, little is known about the origin of the LTR unit. Here, we map the LTR unit and investigate its distribution and morphology across 50,000 km² using a variety of orbital remote sensing data sets. The characteristics of the LTR unit include draping palaeo-topographic surfaces, deposition over a wide elevation range, and a consistent vertical thickness that can be best explained by airfall deposition including a primary or reworked volcanic palaeo-ashfall. Previous research suggests that the LTR unit was not significantly buried, and we find it to be preferentially preserved with a high mechanical strength in discrete deposits representing palaeo-topographic lows. We suggest this could be attributed to localized cementation via upwelling groundwater. This scenario suggests that most of the phyllosilicate-bearing exposures may not have been protected over geologic time, as the uncemented LTR sediment would have easily been removed by erosion. However, our observations indicate that the scarped margins of the LTR unit deposits probably exposed regions of the once protected phyllosilicate-bearing unit. These areas could be key science targets for the ExoMars Rosalind Franklin rover mission.

Martian layered ejecta craters are theorized to form by impacting into an ice-rich crust. The inference that some equatorial layered ejecta craters are Amazonian indicates that ice has persisted in the tropics. However, the detailed spatial and temporal distribution and evolution of this ice remain unknown, which is critical to constraining Mars' global water cycle and climate change over eons. Here we estimate absolute model formation ages for layered and radial (ballistic) ejecta craters to constrain the spatial and temporal distribution of equatorial ice. The assumption is that radial ejecta form where volatiles are not present in significant quantities. Ages are derived from the density of smaller craters superposed on the ejecta blankets. We examined 73 craters in a 30° × 30° area centered at 15°S, 355°E, with 44 layered and 29 radial ejecta. Layered and radial ejecta craters are mixed over distances comparable to their diameters, which represents an unreasonably short length scale for ground-ice emplacement. This, along with the lack of trend with age, supports the suggestion that intermittent low-latitude surface ice—from excursions to high obliquity—could be responsible. Analysis also suggests an increasing proportion of layered ejecta craters with decreasing diameter for those older than 3.4 Ga. This trend would support the hypothesis of more ice being available in early martian history. Conversely, this could indicate that “armoring” preferentially preserves layered ejecta relative to radial ejecta.